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Here `X` is an associated constraint synonym of the class `Coll`. The key point is that differnet instances can give different definitions to `X`. The GHC wiki page describes the design [[http://hackage.haskell.org/trac/ghc/wiki/KindFact WikiConstraint]], and Max's blog posts give more examples [[http://blog.omega-prime.co.uk/?p=61 ConstraintFamlies], [http://blog.omega-prime.co.uk/?p=127 ConstraintKind]].

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Here `X` is an associated constraint synonym of the class `Coll`. The key point is that different instances can give different definitions to `X`. The GHC wiki page describes the design [[http://hackage.haskell.org/trac/ghc/wiki/KindFact WikiConstraint]], and Max's blog posts give more examples [[http://blog.omega-prime.co.uk/?p=61 ConstraintFamlies], [http://blog.omega-prime.co.uk/?p=127 ConstraintKind]].

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* '''Associated type synonym defaults.''' Haskell lets you give a ''default method'' for the operations of a class.